Typical electronic devices are provided with a secondary battery as a drive power source. Such electronic devices include a charging circuit for charging the secondary battery using a charging current supplied from an external power source.
It is desirable that the main unit of an electronic device be able to control an AC adaptor to regulate the output voltage supplied from the AC adaptor to the main device. In this case, the AC adaptor includes a voltage regulating circuit, and a detection circuit included in the main device detects a voltage or the like to produce a control signal for controlling the voltage regulating circuit. Furthermore, since the main device needs to detect whether or not the coupled AC adaptor is suitable for the device, the AC adaptor outputs information identifying the AC adaptor to the main device. For example, the detection circuit of the main device monitors a voltage of the coupling terminal and determines whether or not the coupled AC adaptor is suitable for the device. The main device may not control an AC adaptor which is not suitable for the device, so the main device turns off a switch disposed in the power supply path to protect system circuits and the like of the main device.
In the above electronic device, the AC adaptor is coupled to the main device via, for example, a DC plug 90 illustrated in FIG. 1. In the DC plug 90, a cylindrical power terminal 92 is formed in the outer circumference surface of a cylindrical insulator 91; a cylindrical ground terminal 93 is formed in the inner circumference surface of the insulator 91; and a rod-like control terminal 94 is arranged at the axial center of the insulator 91. The length of the control terminal 94 is set shorter than the lengths of power terminal 92 and ground terminal 93 in a direction of inserting the DC plug 90. That is, the tip ends of the power terminal 92 and ground terminal 93 protrude further relative to the tip end of the control terminal 94. The AC adaptor outputs an identification signal via the control terminal 94 to the main device; and the main device outputs a control signal via the control terminal 94 to the AC adaptor.
As depicted in FIG. 1, the ground terminal 93 and power terminal 92, and then the control terminal 94 are electrically coupled in order to terminals of the main device. With this configuration, the signal (information) is supplied to the detection circuit of the main device after supplying the power supply voltage. Thus, in case the DC plug 90 coupled to the AC adaptor after the AC plug has been inserted in an AC outlet is coupled to the main device, the main device is prevented from being damaged. Such configuration in which coupling of the control terminal 94 is done after coupling of the power terminal 92 and ground terminal 93 is used not only by the DC plug 90 having this shape but also by a DC plug having another shape.
As further depicted in FIG. 1, in the above electronic device, during a period from coupling of the power terminal 92 and ground terminal 93 of the DC plug 90 to coupling of the control terminal 94, the terminal of the main device to be coupled to the control terminal is still not coupled. As a result, the potential of the terminal of the main device may become unstable, causing a malfunction.